The present invention relates to methods of melting metal and, more particularly, to a method of melting aluminum and aluminum alloys in a vertical shaft furnace.
It is well-known to melt ferrous and some non-ferrous metals, such as copper, in vertical shaft furnaces as exemplified by the furnaces disclosed in the following U.S. patents and the patents cited therein:
U.S. Pat. No. 2,283,163 PA1 U.S. Pat. No. 3,199,977 PA1 U.S. Pat. No. 3,715,203 PA1 U.S. Pat. No. 3,759,699 PA1 U.S. Pat. No. 3,788,623 PA1 U.S. Pat. No. 4,129,742 PA1 U.S. Pat. No. 4,243,209 PA1 U.S. Pat. No. 4,311,519 PA1 U.S. Pat. No. 4,315,755 PA1 U.S. Pat. No. 4,375,352
Another known furnace which is said to be useful for melting aluminum is disclosed in U.S. Pat. No. 3,809,378. The furnace disclosed in that patent comprises the combination of a primary melting chamber with a vertical flue and a secondary melting chamber connected to the primary melting chamber. Heat is transferred to the metal in the primary melting chamber by convection where it is "half-melted" using a high velocity burner. Thereafter, the "half-melted" metal flows to the secondary melting chamber where it is completely melted by radiant heat.
Typically, aluminum and aluminum alloys are melted in a reverberatory furnace which differs from a vertical shaft furnace primarily in the manner in which heat is transferred to the aluminum metal. In a reverberatory furnace, heat is transferred to the metal to be melted mainly by radiation from the walls of the furnace, and to a lesser extent, by conduction of heat from molten metal to solid metal. Heat transfer to the metal in a shaft furnace, on the other hand, is primarily by way of convection, only a negligible amount of heat being transferred by either radiation from the furnace walls or by conduction.
In metal melting applications, it is generally known that shaft furnaces are about twice as efficient as reverberatory furnaces in terms of gas consumption rates per unit weight of metal melted in BTU/lb. However, shaft furnaces do not appear to have been utilized to any significant extent in the aluminum industry for melting aluminum and aluminum alloys.
It has been found that one problem associated with melting aluminum or aluminum alloy metals by convection in a shaft furnace using conventional, high velocity burners is the tendency for the low density aluminum metals, especially aluminum in small scrap form, to be "blown" by high velocity gas impingement against the walls of the furnace rather than falling by gravity onto the furnace hearth. In addition, it has been found that molten, semi-molten, and solid aluminum metal can also be "blown" by the high velocity burner gases into other burners and burner openings disposed about the furnace wall, thereby causing furnace inefficiency, potential burner blockages, and significantly increasing furnace maintenance costs.
One way of overcoming the aforementioned problem is to substantially reduce burner velocity. However, melting rate is directly proportional to burner velocity, and it is highly preferred that burner velocity be maximized according to the type and shape of the aluminum material to be melted.
Another way of overcoming the problem of "blowing" the aluminum metal is to utilize the furnace of the aforementioned U.S. Pat. No. 3,809,378, which has only a single high velocity burner directed diametrically across the primary melting chamber toward the opening into the secondary melting chamber. Thus, any molten, semi-molten or solid aluminum metal "blown" across the primary melting chamber by the high velocity burner gases is directed into the secondary melting chamber of reverberatory portion of the furnace where it is subjected to heating and melting under less than optimum heat transfer conditions, i.e., radiant heating in lieu of convection heating.
Another problem associated with melting aluminum metals in a reverberatory furnace is the risk of explosion resulting from moisture contamination of the metal charged to the furnace. Should any moisture be entrapped in the metal when it is charged to a hot furnace containing a molten pool of aluminum, i.e., a "wet" hearth, the moisture is likely to flash into steam with a resulting expansion in volume that may cause a potentially dangerous explosion. The possibility of such an explosion in a shaft furnace is highly remoted because a shaft furnace is typically a "dry" hearth furnace and because the metal is charged to the furnace at the top of the shaft where it is preheated by convection, which advantageously evaporates all moisture from the charge.